To be completely honest, I was surprised at Marriott's announcement of the 4pm guarantee when it first launched. Just for that simple reason: I can check in at 4pm, check out at 4pm, and I've certainly stayed in lot of business hotels where there are a lot of elites. The only thing I can surmise is that Marriott has all the hard data - most elites don't actually stay until 4pm, and most don't actually show up right at 4pm, so there ends up being enough time to clean enough rooms to make everybody happy.

In practice, it seems to work. It's incredibly rare that I've showed up at check-in time and they've had no available rooms for me. Sometimes they'll ask if I'm willing to wait for an upgrade...to which I say yes...but I've gotten the sense that if I badly wanted into a standard room at the exact check-in time they could have accommodated that.